'Weekly Problem 4 - 2006' printed from http://nrich.maths.org/

A roll of adhesive tape is wound round a central cylindrical core
of radius $3 \; \text{cm}$. The outer radius of a roll containing
$20 \; \text{m}$ of tape is $4 \; \text{cm}$. What is the outer
radius of a roll containing $80 \; \text{m}$ of tape?